The present invention relates to a novel two-step method for treating a liquid stream contaminated with an iodine-containing compound using a pretreatment step in combination with an adsorption step. The adsorption step is performed by contacting the liquid stream with a solid adsorbent material comprising a zeolite having a silica to alumina molar ratio from about 5 to less than 15 which has been cation exchanged with a metal selected from the group consisting of silver, mercury, copper, lead, thallium, palladium or mixture thereof.
Methanol carbonylation, the reaction of methanol with carbon monoxide, is used to produce a significant share of the world""s acetic acid and represents the basis for virtually all new acetic acid capacity. The fundamental process, whereby methanol and carbon monoxide are reacted in the presence of a rhodium catalyst and methyl iodide promoter, is disclosed in U.S. Pat. No. 3,769,329 B1 and has become well-known as the xe2x80x9cMonsanto processxe2x80x9d. Although numerous improvements have since been developed, the use of an iodine-containing promoter, either as an organic iodide or metal iodide salt, has proven necessary to obtain industrially-competitive reaction rates and production economies.
Unfortunately, the use of any suitable iodine-containing promoter invariably results in the incorporation of trace iodine and organic iodide impurities into the final acetic acid product. These contaminants result from numerous transformations (thermal cracking, recombination, isomerization, etc.) of the iodine-containing catalyst promoters, which occur not only in the reactor, but also in downstream equipment, such as distillation column reboilers and recycle lines. A resulting array of C1 to C10 organic alkyl iodide species is produced, which are removed from the acetic acid product with varying degrees of effectiveness via the standard distillation steps used in down stream purification. Additionally, iodine may be present in the acetic acid product in the form of hydrogen iodide or iodide salts. Ultimately, without supplemental treatment to remove trace iodine-containing contaminants, product acetic acid made using methanol carbonylation technology with even the most careful fractionation steps, will still contain a small amount, typically below 100 parts per billion (ppb) of total iodine (both organic and inorganic) by weight.
The interest in a process for essentially complete removal of iodine-containing contaminants from acetic acid stems from the large share (about 40 to 50%) of its use as a precursor for vinyl acetate monomer (VAM) synthesis. Current methods of VAM production rely on a catalyst which is intolerant to even minute levels of iodine-containing compounds in the acetic acid feedstock. Therefore, the VAM production costs associated with reduced catalyst life increase dramatically with increasing feed iodine concentration.
Several disclosures in the prior art present techniques for the selective removal of iodine-containing species from process streams such as nuclear reactor containment environment off gases as well as emissions from spent nuclear fuel reprocessing operations. For example, U.S. Pat. No. 3,658,467 B1 addresses the removal of radioactive iodine-containing materials from the gaseous waste streams generated either during normal nuclear fuel reprocessing operations or even in the event of a fuel element cladding failure whereby radioactive methyl iodide is formed in significant amounts. The solution proposed in the ""467 patent is a zeolite X molecular sieve exchanged with silver for treating the gaseous waste stream. All cited examples referring to the adsorptive ability of this formulation are based on performance in a dry air stream contaminated with trace radioactive methyl iodide. The structures of X-type zeolites are known to have aluminosilicate frameworks with maximum silica to alumina molar ratios, expressed as the ratio of SiO2 to Al2O3 in the fundamental zeolite framework, of about 3 and pore openings typically in the range of 7 to 8 xc3x85.
In U.S. Pat. No. 4,735,786 B1, an alternate solution for filtering radioactive iodine-containing compounds from nuclear facility exhaust gases in the event of an accident is proposed. In offering an improvement over the prior art, the ""786 patent recognizes the practical deficiencies of silver-exchanged zeolite X adsorbent for this service under high humidity conditions. The improvement offered is a different type of adsorbent, characterized as a high silica to alumina molar ratio pentasil zeolite. The adsorbent specified is exemplified by the well-known ZSM-5 type material, which is clearly described in U.S. Pat. No. 3,702,886 B1 as having ten-member rings forming medium-sized pores in the range of 5.1 to 5.6 xc3x85. The teachings and specific examples of the ""786 patent are restricted to pentasil zeolites having silica to alumina molar ratios in the range of 15 to 100, preferably 20 to 50.
In U.S. Pat. No. 4,913,850 B1, another solution for methyl iodide removal from gaseous streams is presented, whereby a silver-exchanged xe2x80x9cbinderlessxe2x80x9d zeolite material, composed of 80 to 90% zeolite X and 10 to 20% zeolite A, is used. Among the possible candidates for zeolite X materials, those having the faujasite structure are of particular interest. As mentioned previously, zeolite X formulations generally have a maximum silica to alumina molar ratio of 3. In U.S. Pat. No. 5,075,084 B1, the progress of treating radioactive iodine-containing gas streams is continued, where the problem of the proposed silver-exchanged zeolite material catalyzing the highly exothermic reaction of hydrogen and oxygen and, in the extreme case, causing catalytic ignition of hydrogen, is solved. According to the ""084 patent, this undesired side reaction is suppressed when a heavy metal such as lead is added to the silver-exchanged adsorbent. The underlying zeolite compositions of the preferred materials in this patent and the previously mentioned ""850 patent are identical.
In U.S. Pat. No. 4,088,737 B1, gaseous radioactive methyl iodide removal is further addressed in a multi-step treatment procedure where the initial gas purification is performed with a silver-exchanged zeolite exemplified by zeolite X. After iodine-compound breakthrough, regeneration and concentration steps are undertaken, which involve i) withdrawing the spent adsorbent from contact with the gaseous waste stream, ii) subjecting the adsorbent to desorption conditions with a hydrogen-rich stream to produce a hydrogen iodide containing off gas, and iii) treating this effluent gas with a lead-exchanged zeolite to re-adsorb and concentrate the desorbed hydrogen iodide. Lead-exchanged zeolite X is specifically cited as achieving the desired result for the final adsorption step. The advantage of the multi-step treatment is that the long-term storage of the contaminated material is less expensive for the lead-exchanged zeolite, compared to a silver-exchanged material.
In spite of these continuing developments and improvements in trace iodine and organic iodide removal from gaseous effluent streams, the methods employed have been found unsuitable for the more difficult problem of iodine-containing compound adsorption from corrosive liquids, such as commercial acetic acid product streams. Adsorbent carrier materials of the prior art such as zeolite X and zeolite A, which are classified as having low silica to alumina molar framework ratios (typically below 5), have experimentally been proven to be unstable in acetic acid. This means that the dissolution (or leaching) rate of framework components into the liquid is sufficiently large to render such materials ineffective for iodine-containing compound adsorption service in corrosive liquid media. Depending on the specific silica to alumina framework molar ratio, the pentasil zeolites, exemplified in prior art gas-phase iodine-containing compound removal using ZSM-5, are significantly more stable in acetic acid than zeolite types X and A. However, the pore sizes of pentasil zeolites, as determined by their molecular aluminosilicate crystal channel width, are too small to effectively allow passage of the straight- and branched-chain C3 to C8 alkyl iodides which are generally present as contaminants in commercial acetic acid product streams. In contrast, the iodine-containing compounds present in industrial nuclear power plant waste gases are normally radioactive molecular iodine and methyl iodide only.
Other teachings more specifically apply to iodine-compound removal from corrosive liquid media, where the principal area of concern, as described previously, is in the manufacture of carboxylic acids such as acetic acid via a process which results in a product stream contaminated with trace amounts of iodine-containing byproducts. To achieve the extremely low levels of iodine-containing compounds demanded industrially, significant emphasis has been placed on the development and utilization of solid materials capable of adsorbing essentially all iodine-containing compounds from acetic acid streams.
For instance, in U.S. Pat. No. 5,457,230 B1, the use of activated carbon fiber is contemplated for this purpose. However, the examples demonstrate the removal of molecular iodine and hydrogen iodide only and fail to specifically disclose the level of iodine-containing compounds in the treated acetic acid stream. In the case of iodine-compound removal from acetic acid, it is the ability of the invention to provide a treated product with only extremely minute levels of total iodine which primarily determines its practical utility. It is known in the art that activated carbon alone can neither remove iodine-containing compounds from commercial acetic acid streams to single parts per billion levels, nor can it effectively remove organic iodide species, such as methyl iodide and hexyl iodide which are commonly present in these product streams, without the use of an iodine-reactive metal.
Recently, considerable development efforts in acetic acid purification technology have focused on resins containing iodine-reactive metals such as silver, mercury, copper, lead, thallium, palladium or combinations of these metals known to react with iodine-containing compounds to form insoluble complexes. For example, in U.S. Pat. No. 4,615,806 B1, the removal of these impurities is achieved with a macroreticulated strong acid cation-exchange resin which is stable in the organic medium and has at least one percent of its active sites converted to the silver or mercury form, presumably by cation-exchange. The use of macroreticulated resins is claimed as an advance over the prior art formulations, which are generally characterized as gel-type ion-exchange resins, for this service. In U.S. Pat. No. 5,139,981 B1, other silver-exchanged resins are offered, along with a novel technique for preparing such resin compositions. In U.S. Pat. No. 5,220,058 B1, a performance benefit is claimed, whereby the subject resin contains thiol functional groups, compared to the prior art sulfonate functional groups, which are exchanged with the iodine-reactive metal. In U.S. Pat. No. 5,227,524 B1, the resin degree of crosslinking is decreased somewhat, resulting in improved silver utilization. In U.S. Pat. No. 5,300,685 B1, the iodine-reactive metal is coordinated, as a salt, with a polymeric resin, rather than being ionically bound to a cation-exchange resin. In U.S. Pat. No. 5,344,976 B1, a resin guard bed without the iodine-reactive metal is placed upstream of the metal-exchanged resin to scavenge any metal cations in the acetic acid stream that would otherwise potentially displace the iodine-reactive metal. Finally, in U.S. Pat. No. 5,801,279 B1, an improved method of operating the iodine-compound removal step is disclosed in order to reduce the amount of leaching of the iodine-reactive metal into the treated acetic acid effluent stream. As noted in this reference, the dissolution of the iodine-reactive metal is acknowledged as a problem for iodine-compound removal techniques of the prior art whereby metal-exchanged resins are applied.
While the invention of U.S. Pat. No. 4,615,806 B1 and other modified resin-based formulations have been used commercially with some success, resins in general suffer some disadvantages, in addition to the previously-mentioned metal loss, when used in the acetic acid environment of the present invention. More specifically, resins, even those characterized as xe2x80x9cstablexe2x80x9d are known to xe2x80x9cswellxe2x80x9d or increase in diameter by as much as 50% when exposed to an organic medium, making bed design difficult. Resins are also vulnerable to decomposition at relatively mild conditions and are furthermore susceptible to chemical attack by corrosive reagents. These factors additionally complicate the use of a resin-based material for the purification of acetic acid.
Also associated with the application of resins in this service is a narrowly-limited range of acceptable operating temperatures due to decomposition, softening, loss of strength, or other detrimental structural changes resulting from thermal effects. Typically, resins begin to chemically decompose at 100 to 200xc2x0 C., resulting in destruction of their fundamental networks and ion-exchange sites. For example, the preferred resin of the ""806 patent is essentially a sulfonated copolymer of styrene and divinylbenzene, and at relatively mild temperatures the acid exchange sites are susceptible to acid-catalyzed desulfonation which leads to release of not only metal cations but also sulfur-containing compounds into the liquid effluent stream. Such materials interfere with further chemical processing of this product. The ""806 patent is silent regarding any regeneration or reactivation method because these steps would undoubtedly require temperatures that the macroreticulated resin taught therein cannot withstand without substantial degradation.
As noted in U.S. Pat. No. 5,801,279 B1, operation of the iodine-compound removal step in an acetic acid medium at elevated temperature is beneficial in terms of improving the rate of the desired reaction, which leads to the formation of insoluble metal iodides. However, the resin-based materials traditionally employed for the treatment of acetic acid streams are generally incompatible with high-temperature operation.
A final consideration regarding cation-exchanged resins which are known in the art to adsorb trace iodine-containing compounds from liquids is the considerable expense of such materials, associated with the use of valuable iodine-reactive metals (e.g. silver or mercury) incorporated into these formulations. This concern for cost is evidenced by ongoing efforts in industry to most judiciously expend these metals by ensuring their reaction with only those iodine-containing compounds (e.g. alkyl iodides) that cannot be removed through less expensive, conventional means in a pretreatment step.
For example, in U.S. Pat. No. 5,155,265 B1, a pretreatment is offered with the intent to reduce the iodine loading on the metal exchanged resin. This method entails contacting an iodine-compound contaminated feed with ozone to oxidize the most reactive of the impurities (which also include carbonyl compounds) and thereby increase the total iodine compound removal that can be achieved using activated carbon, prior to the final treatment with a silver-exchanged resin. As noted in U.S. Pat. No. 5,457,230 B1, activated carbon alone can be useful for pretreatment purposes, based on its capacity for the adsorption of molecular iodine and hydrogen iodide. This pretreatment medium thus allows for a more selective use of metal exchange sites of the final adsorbent for the removal of only the most unreactive iodine-containing compounds. Other materials that do not comprise an iodine-reactive metal but nevertheless demonstrate a capacity for the removal for at least some iodine-containing compounds (and are thus suitable for pretreatment) include various anion exchange resins.
In U.S. Pat. No. 5,576,458 B1, a pretreatment to significantly reduce quantities of hydrogen iodide in the feed stream is disclosed, also in the context of purifying acetic acid. Most commercial acetic acid products, prior to final purification, contain hydrogen iodide in significant excess of alkyl iodides. An efficient method to remove this impurity comprises injecting methanol into the commonly used dehydration column designed for the removal of contaminant water in an overhead stream. Methanol reacts with hydrogen iodide to form methyl iodide and water, and both of these products are then separated from the acetic acid in a light fraction. Of the residual, unreacted hydrogen iodide remaining in the acetic acid, a significant portion can be further removed through subsequent reaction with a salt or base (e.g. potassium hydroxide). This step of the pretreatment results in the formation of an iodide salt (e.g. potassium iodide) which is then separated in the heavy fraction of a final distillation column, used in general in acetic acid production to separate high boiling components such as propionic acid.
Overall, the method is advantageous for removing the bulk of the hydrogen iodide and thereby preventing this component, often the most prevalent iodine-containing impurity, from quickly consuming the reactive sites of the final adsorbent. Careful consideration must be given to the amount of base added for the conversion of hydrogen iodide to iodide salt. Base injection in significant excess of the amount required to neutralize the hydrogen iodide impurity will ultimately consume acetic acid product. For example, excess potassium hydroxide will react with acetic acid to form potassium acetate salt. Of course, it is possible to employ various combinations of the aforementioned ozone treatment, adsorption, distillation, methanol injection, and neutralization steps for pretreatment of a liquid stream contaminated with an iodine-containing compound.
An alternative type of pretreatment involves the removal of contaminant metals in the form of metal cations that are also normally present in the iodine contaminated feed stream of the present invention. These metals originate mainly from the metallic catalysts and catalyst promoters used in upstream conversion (e.g. methanol carbonylation) operations. Metals are also present to some extent due to the corrosion of materials used for the production plant.
Thus far, therefore, the prior art has offered several techniques for the removal of both iodine-containing impurities and contaminant metals. These methods, while they fail in isolation or in combination to achieve the extremely low levels of iodine-containing compounds demanded industrially, are valuable in many cases for pretreatment to remove particular contaminants prior to a final adsorption step. Thus, the efficiency of the adsorbent used in this step is maximized in terms of separating only the most non-reactive iodine-containing compounds, which are usually present in very small quantities. Nevertheless, regardless of the pretreatment, the effectiveness of the treating method overall is dependent on the performance of the final adsorbent for removing trace impurities such as methyl and hexyl iodide to single ppb levels or below. It is understood hereafter that trace iodine contamination levels are expressed in terms of the total weight of iodine relative to the weight of the liquid stream in which iodine contaminants are present.
The problem therefore addressed by the present invention is to provide a method comprising both a pretreatment and an adsorption step for the essentially complete removal of iodine-containing impurities from liquids such as commercial acetic acid product streams. At least a portion of these impurities, or at least a portion of a metal contaminant, is removed in the pretreatment step, while at least a portion of the residual iodine-containing compounds not removed in this step are subsequently adsorbed in the adsorption step. The invention is further characterized in that the adsorbent used in the final adsorption step is free of the substantial temperature restrictions, chemical exposure effects, and swelling problems associated with the typical organic resin materials used in the prior art. Another feature of the adsorbent is that it may be conveniently reactivated by contacting it with a solution of iodine-reactive metal cations when the originally loaded metal becomes deactivated after reaction with iodine-containing impurities.
Furthermore, because the adsorbent can be subjected, without any undue performance deficit, to a significantly wider range of conditions than those of the prior art, the invention also provides a means for adsorbent regeneration through contact with a regenerant gas stream at elevated temperature. Such a regeneration is described in detail in U.S. Pat. No. 4,088,737 B1 for zeolite-based adsorbents used in the well-known process of iodine-containing compound removal from gas streams. The regeneration entails subjecting the spent adsorbent to a stream of hot gas comprising hydrogen.
Significant teachings in the prior art associated with the use of non-resin adsorbents actually point away from their utility in the treatment of corrosive liquid media. In particular, in the comparative example recited in U.S. Pat. No. 4,615,806 B1 (column 6, lines 36 to 49), a silver-exchanged zeolite, characterized as {fraction (1/16)} inch 5A molecular sieve pellets, was tested in acetic acid for contaminant methyl iodide removal and found to be unstable as evidenced by the continuous silver leaching from the adsorbent and the finding of a yellowish precipitate in the treated effluent. Given this discouraging result, it is remarkable that a suitable inorganic adsorbent for use in this corrosive environment has been discovered.
The adsorbent material in fact comprises a zeolite that has been cation exchanged with a metal known to react with iodine-containing compounds, present in trace amounts in the feed stream of the present invention. This finding of an inorganic material suitable for the treatment of a corrosive acetic acid feed stream is associated with the realization that zeolites with sufficiently high silica to alumina molar ratios are indeed stable in this service. The silica to alumina molar ratio, of course, refers to the composition of the fundamental three dimensional network structure which characterizes the zeolite. It is actually this variable, rather than the type of zeolite itself, which determines its ability to withstand corrosive liquid environments. Experimentally, good results were obtained with silica to alumina molar ratios above about 5, with better results obtained at ratios above about 6.5, and superior results obtained at ratios above about 8. The upper bound of the silica to alumina molar ratio is based on the amount of ion exchange sites available for loading of a suitable iodine-reactive metal (e.g. silver). In contrast to the teachings of the aforementioned U.S. Pat. No. 4,735,786 B1, zeolites having silica to alumina molar ratios of less than 15 are in fact useful for iodine-compound removal. Such zeolites have been found sufficiently stable in acidic media and also in possession of adequate ion exchange capacity for effective contaminant removal to parts per billion levels.
A further unexpected finding was that such a silica-rich zeolite, when used in iodine-containing compound adsorption service of the present invention, can be reactivated using a relatively simple procedure and also regenerated at high temperatures when necessary.
In one embodiment the present invention is a process for treating a liquid feed stream containing a contaminant comprising an iodine compound, the process comprising 1) pretreating the liquid feed stream to remove at least a portion of the contaminant and yield a pretreated liquid feed stream containing at least a residual amount of the iodine compound, and 2) contacting the pretreated liquid feed stream with an adsorbent comprising a zeolite having a silica to alumina molar ratio from about 5 to less than 15 which has been cation exchanged with a metal selected from the group consisting of silver, mercury, copper, lead, thallium, palladium, and mixtures thereof, at adsorption conditions to adsorb at least a portion of residual amount of iodine-compound to yield a treated liquid stream.
In a more specific embodiment the pretreating step of the present invention as described above comprises 1) contacting the liquid feed stream with methanol to convert at least a portion of the iodine compound to methyl iodide and yield a methyl iodide-containing liquid stream, 2) fractionating the methyl iodide-containing liquid stream to separate at least a portion of the methyl iodide therefrom in an overhead liquid stream and yield a bottoms liquid product stream containing at least a portion of the iodine compound, 3) contacting the bottoms liquid product stream with a salt or a hydroxide compound of a cation to convert at least a portion of the iodine compound therein to an iodide salt and thereby yielding an iodide salt-containing liquid stream, 4) fractionating the iodide salt-containing liquid stream to separate at least a portion of the iodide salt therefrom in a heavy ends liquid stream and yield a light ends liquid product stream, and 5) contacting the light ends liquid product stream with a pretreatment medium to provide the pretreated liquid stream.
A secondary object of the present invention is to provide the process as described above, where the process is carried out until the adsorbent has substantially reached its adsorption capacity, at which point the adsorbent is reactivated by contacting it with a solution of a salt of a reactivation metal where the metal is selected from the group consisting of silver, mercury, copper, lead, thallium, palladium, and mixtures thereof such that an amount of reactivation metal is added to the adsorbent.
Still another object of the present invention is to provide the process as described above, where the process is carried out until the adsorbent has substantially reached its adsorption capacity, at which point the adsorbent is contacted with a regenerant gas stream comprising hydrogen at conditions effective to strip substantially all of the adsorbed iodine as hydrogen iodide to yield a regenerated adsorbent.
Other objectives and embodiments are associated with the various preferred procedures and features connected with the invention and are discussed in the following detailed description.